Oil shale reserves constitute a major alternative for rapidly dwindling petroleum reserves. In the United States, it has been estimated that there are well over two trillion barrels of untapped oil locked in oil shale rock. Also, trapped beneath thick layers of shale are enormous amounts of methane in tightly trapped, isolated pockets of gas. For example, in the Appalachian Basin, the gas trapped in Devonian Shale is estimated to be as high as 500 quadrillion cubic feet which far exceeds the total of all other potential natural gas sources, both onshore and offshore. However, access to the oil content of shale and to the trapped natural gas is not readily attained. At the present time there is no feasible process for fast release of the entrapped gas. Shale is a carbonaceous rock whose organic content is substantially kerogen, which is a high-molecular weight organic polymer precursor of oil. One can produce oil from shale by heating to pyrolysis temperatures of 427.degree. to 538.degree. C. The inorganic matrix is usually comprised of dolamite, calcite, quartz and clays. While oil is obtainable from shale using current technology, the available processes are generally energy-intensive particularly those processes which attempt to increase conversion efficiency by crushing the shale down to successively smaller sizes to expose more surface to applied heat.
Experiences with other mining operations have not been very helpful. For example it has been known to chemically fracture such materials as coal using a variety of materials including various amino compounds such as ammonia. However, one would hardly expect that such techniques would be successful with oil shale. Whereas coal is a relatively soft and porous material, oil shale is a very fine-grained sedimentary rock. Permeability is measured in darcys, a unit of porous permeability equal to the permeability of a medium through which the rate of flow of a fluid having 1 centipoise viscosity under a pressure gradient of 1 atmosphere per centimeter would be 1 cubic centimeter per second per square centimeter cross section. Whereas coal has a permeability of about 1000 darcys, shale has a permeability of less than 10.sup.-5 darcys, a hundred million-fold difference. Furthermore, whereas coal is 90% organic, oil shale is 80% or more inorganic.
The present invention provides a method for treating oil shale to induce fracturing of the shale and resides in the discovery that when oil shale has high organic content and/or shallow burial, depending upon the particular shale, one can fracture the shale with liquid or gaseous ammonia or hydrazine as such or in hydrate form. Suitability of a particular shale will depend upon the nature of both the organic and inroganic composition of the shale and cannot be detailed with great precision. Some shale samples are unaffected by the above two amino compounds, as will be amplified on hereinafter. Nevertheless, the process should be applicable to oil shales containing 10 weight percent organic content or more and/or those shales buried at a depth of less than 1500 feet. One can determine the suitability of a particular shale for treatment in accordance with the present invention by conducting an experiment as shown by the examples to be described hereinafter.
More specifically, the present invention provides a method of treating oil shale, having high organic content and/or shallow burial whereby to induce fracturing, comprising the step of contacting the oil shale with an effective amount of an amino compound for a time sufficient to fracture the oil shale. The amino compound can have the formula RNH.sub.2 in which R is hydrogen, .dbd.NH.sub.2 or an aliphatic, aromatic or cyclic hydrocarbon of up to 12 carbon atoms. Ammonia, preferably anhydrous, is the compound of choice, in gaseous or liquid form. The shale can be distinguished from other organic-containing minerals, such as coal, by reference to its low gas permeability, less than 10.sup.-5 darcys, and high inorganic content, as will be discussed in more detail hereinafter.